The Canadian government is evacuating three scientists from Sierra Leone, where their role diagnosing blood samples was crucial to the operation of one of the world’s biggest Ebola treatment centres.

The Canadians were working at a treatment centre operated by Médecins Sans Frontières (MSF) in Kailahun, the eastern district at the heart of the Ebola outbreak in Sierra Leone. The “mobile” laboratory was deployed to West Africa in the spring at the request of the World Health Organization and three teams have now travelled to Sierra Leone from Winnipeg.

The sudden departure of the Canadians is part of a larger evacuation ordered by the World Health Organization, which recently removed its entire team from Kailahun. The decision came after a Senegalese epidemiologist working for it tested positive for Ebola over the weekend, marking the first time a WHO-deployed expert has been infected with the deadly virus.

“This was the responsible thing to do. The field team has been through a traumatic time with this incident,” said Dr. Daniel Kertesz, the WHO’s representative in Sierra Leone, in a written statement. “They are exhausted from many weeks of heroic work, helping patients infected with Ebola. When you add a stressor like this, the risk of accidents increases.”

The three Canadian scientists and the WHO epidemiologist were all living at the same hotel in Kailahun, where staff from MSF were also staying. The hotel has also been accommodating volunteers with the WHO and Red Cross – as well as visiting journalists – but MSF said it is now restricting access to its own staff only.

The Public Health Agency of Canada said the risk is low that any of the Canadian scientists could be infected. None had any contact with sick individuals and they are not showing any signs or symptoms. All three will remain in voluntary isolation, however, and be closely monitored as they make their way home and after they return to Canada.

MSF said it is also investigating which of its own staffers may have come into contact with the WHO epidemiologist, but as of Monday none were showing any signs or symptoms. Ebola patients are only infectious when they are symptomatic and the virus can only be spread through bodily fluids, such as blood, urine, sweat or tears.